1. Field of the Invention
The present invention relates to an angular rate sensor for use in a gyroscope or the like.
2. Description of the Prior Art
The recent computer technology development has resulted in various computerized commercial products with many functions. There has been a growing demand for various sensors for use with such computer-controlled devices. Among these sensors is an angular rate sensor which finds a wide variety of applications such as electronic navigation systems, bearing detectors for robots, stabilizers for actuators, or the like. Angular rate sensors will be required to be smaller in size and yet have higher performance accuracy.
Heretofore, inertial navigation systems with gyroscopes have mainly been used to perform dead reckoning, i.e., to determine the position of a moving object such as an airplane or a ship. While the gyroscopic inertial navigation systems can determine stable dead reckoning positions, they are large in size and highly costly because they are mechanical in nature. Therefore, they cannot be applied to consumer equipment which should be relatively small in size and low in cost.
There has been proposed a vibratory gyro as disclosed in Japanese Patent Application No. 59-55420. According to the disclosed vibratory gyro, a mass is vibrated without any spinning force being applied thereto, and an angular rate of the mass is detected on the basis of Coriolis force which is produced when the angular rate is developed. The disclosed vibratory gyro may be regarded as a vibration sensor having a tuning-fork structure. The principles of the vibratory gyro are disclosed in U.S. Pat. No. 2,544,646.
According to the principles disclosed in U.S. Pat. No. 2,544,646, a rectangular elastic drive (excitation) element and a rectangular elastic detection element are linearly joined to each other, the drive and detection elements lying orthogonally to each other. When the detection element is vibrated at a velocity (V), a force, known as the Coriolis force, acts on the detection element and is detected to determine an angular rate of the detection element.
FIG. 1 of the accompanying drawings illustrates one conventional angular rate sensor and a drive circuit arrangement for the angular rate sensor. The angular rate sensor includes a pair of parallel piezoelectric bimorph drive elements 103, 104 and a pair of parallel piezoelectric bimorph detection elements 101, 102. The drive and detection elements 103, 101 and the drive and detection elements 104, 102 are joined by respective joint members 105, 106 parallel to a nodal axis and lie in orthogonal planes. The drive elements 103, 104 have lower ends joined to each other by a support 107. Thus, the angular rate sensor is in the form of a tuning-fork vibratory device. The vibratory device is resiliently mounted on a base 109 by a metallic elastic member 108 which is joined to the support 107. In operation, the drive elements 103, 104 are vibrated as a turning fork, and when an angular rate is developed on the detection elements 101, 102, an output signal indicative of the angular rate is produced.
The drive circuit for the angular rate sensor operates as follows: When a drive signal is applied from a driving circuit 115 to the drive element 103 to vibrate the same, the other drive element 104 resonates with the drive element 103. Therefore, the drive elements 103, 104 start tuning fork vibration. An amplitude signal from the drive element 104 is detected as a monitor output signal by a driving information extracting circuit 112. The monitor output signal is compared with a reference voltage 113 by an automatic gain control circuit 114, which feeds back its output signal to the driving circuit 115, thereby maintaining the amplitude of the tuning fork vibration at a constant level.
Utilizing a phase signal from the drive element 103, the detection elements 101, 102 produce a sense output signal, which is detected by a detecting circuit 110 in synchronism with a drive phase signal from the driving information extracting circuit 112. An output signal from the detecting circuit 110 is applied to a filter 111 by which an angular rate signal is extracted. In this manner, an angular rate force applied to the angular rate sensor is detected.
Because of irregularities of dimensional accuracy and element uniformity, unwanted spurious radiations or emissions are developed when the vibration is transmitted from the drive elements 103, 104 to the detection elements 101, 102. Leads connected to the detection elements 101, 102 and supports for the leads tend to develop parasitic oscillations and higher-order oscillations. If the tuning fork vibration is not properly balanced, then undesired vibrations are developed in the entire tuning-fork gyro angular rate sensor. As a consequence, the output signal of the angular rate sensor is subjected to drifts as shown in FIG. 2. The drift characteristic, which is one of the important performance factors of the angular rate sensor, is therefore adversely affected.
Besides the irregularities of dimensional accuracy and element uniformity, the structural characteristics of the angular rate sensor itself are also responsible for the generation and propagation of unwanted small vibrations, and hence for output signal drifts. Therefore, it has been difficult for the conventional vibratory-gyro angular rate sensor to detect angular rates with higher accuracy owing to its own dimensional and structural limitations.